Remove qt_mac_applicationIsInDarkMode in favour of QStyleHints::colorScheme()

Now that we have the information available in QStyleHints we don't
need to expose it as a dedicated helper function, and definitely not
in QtCore.

Change-Id: If5ad3991d78810457b51a30aadc3bf3af593940c
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/kernel/qcore_mac.mm

@@ -323,13 +323,6 @@ QDebug operator<<(QDebug debug, const QCFString &string)
 #endif // !QT_NO_DEBUG_STREAM
 
 #if defined(Q_OS_MACOS) && !defined(QT_BOOTSTRAPPED)
-bool qt_mac_applicationIsInDarkMode()
-{
-    auto appearance = [NSApp.effectiveAppearance bestMatchFromAppearancesWithNames:
-            @[ NSAppearanceNameAqua, NSAppearanceNameDarkAqua ]];
-    return [appearance isEqualToString:NSAppearanceNameDarkAqua];
-}
-
 bool qt_mac_runningUnderRosetta()
 {
     int translated = 0;

src/corelib/kernel/qcore_mac_p.h

@@ -185,7 +185,6 @@ private:
 };
 
 #ifdef Q_OS_MACOS
-Q_CORE_EXPORT bool qt_mac_applicationIsInDarkMode();
 Q_CORE_EXPORT bool qt_mac_runningUnderRosetta();
 Q_CORE_EXPORT std::optional<uint32_t> qt_mac_sipConfiguration();
 #ifdef QT_BUILD_INTERNAL

src/plugins/platforms/cocoa/qcocoatheme.mm

@@ -505,7 +505,10 @@ void QCocoaTheme::requestColorScheme(Qt::ColorScheme scheme)
 */
 void QCocoaTheme::updateColorScheme()
 {
-    m_colorScheme = qt_mac_applicationIsInDarkMode() ? Qt::ColorScheme::Dark : Qt::ColorScheme::Light;
+    auto appearance = [NSApp.effectiveAppearance bestMatchFromAppearancesWithNames:
+            @[ NSAppearanceNameAqua, NSAppearanceNameDarkAqua ]];
+    m_colorScheme = [appearance isEqualToString:NSAppearanceNameDarkAqua] ?
+        Qt::ColorScheme::Dark : Qt::ColorScheme::Light;
 }
 
 Qt::ContrastPreference QCocoaTheme::contrastPreference() const